This application is based on patent application No. 11-72522 filed in Japan, the contents of which are hereby incorporated by references.
This invention relates to an apparatus and method for measuring a three-dimensional shape of an object such as a machine part in a non-contact manner.
Conventionally, optical measuring methods have been known as means for measuring a three-dimensional shape of an object in a non-contact manner. These methods are roughly classified into four categories as follows.
First methods are so-called xe2x80x9clight Cutting Methodxe2x80x9d which adopt the trigonometric measuring method as a basic principle and measure a three-dimensional shape of a measurement object by detecting a direction and a distance from a light projecting section to the measurement object. These methods enable a relatively highly accurate measurement with a simple construction comprised of the light projecting section and a light receiving section. On the other hand, they have disadvantages of a dead spot in measurement depending on the shape of the measurement object and a likeliness to be influenced by the surface condition of the measurement object.
Second methods measure a three-dimensional shape of a measurement object by projecting a specified regular optical pattern to the measurement object and detecting a variation degree of the optical pattern. These methods also have a disadvantage of a large dead spot in measurement despite its advantage of being capable of a highly accurate measurement with a simple construction.
Third methods measure a three-dimensional shape of a measurement object by projecting a ray to a measurement object as a radar does, calculating a distance to the measurement object by calculating a time required for reflected light to reach a light receiving section, and measuring the three-dimensional shape based on the calculated distance and the light projecting position. Although these methods are suited to measuring a long distance, they are not suited to measuring a shape of a small object, such as mechanical parts for a small machine, with high accuracy.
Fourth methods measure a three-dimensional shape of a measurement object by projecting a ray to a measurement object, calculating a distance to the measurement object by detecting a peculiar response of reflected light at a focus of an optical system, and measuring the three-dimensional shape based on the calculated distance and the light projecting position. These methods include a knife-edge method, an astigmatism method, and a confocal method.
The knife-edge method takes advantage of the inversion of, for example, the shadow of the edge of a knife across a focus in a detector. The astigmatism method takes advantage of a change of oval spot light in orientation across a focus using an optical system having an astigmatism.
According to the confocal method, as shown in FIG. 14, a measurement object 100 is moved within a specified range of a focus of a confocal optical system 101, first and second pinholes 102 and 103 are arranged in conjugated positions of the focus, an illumination ray emitted from a light source 104 through the first pinhole 102 is focused by the confocal optical system 101 and projected onto the measurement object 100 to be reflected or diffused, and a ray reflected on the measurement object 100 is caused to transmit through the confocal optical system 101 again and is detected by a light receiving section 105 provided immediately after the second pinhole 103.
Since the fourth methods are based on the change of an optical response which occurs before and after the focus position, measurement can be conducted with a higher resolution as compared with the first to third methods. Further, the confocal method can conduct measurement with a considerably high resolution since the light is most strongly observed in the light receiving section when the surface of the measurement object 100 corresponds with the focus of the confocal optical system 101.
According to the first to third methods, information of shape is obtained when the measurement light reflected on the measurement object is received. On the contrary, according to the fourth methods, shape information cannot be obtained until the measurement object is entirely scanned and a position where the intensity of the measurement light is at maximum is detected. Accordingly, the fourth methods disadvantageously require a longer time than the first to third methods. The fourth methods have additional disadvantages of a complicated optical system and a necessity for a highly precise optical design in order to suppress an aberration to a lower level.
Further, in a conventional confocal detection method shown in FIG. 14, the optical aberration doubly influences in the fourth methods since the light emitted from the light source 104 transmits through the confocal optical system 101, is reflected and diffused on the surface of the measurement object 100, and is received by the light receiving section 105 after transmitting through the confocal optical system 101 again. Therefore, the optical aberration doubly influences.
FIG. 15 shows a light intensity distribution (dotted image distribution) when the light from the light source 104 transmits through the optical system 101 having a certain wave aberration once. FIG. 16 shows a light intensity distribution (dotted image distribution) when the light having transmitted through the optical system 101 is regularly reflected on the measurement object 100 and then transmits through the optical system 101 again, i.e., when the light transmits through the optical system 101 twice. A peak of the dotted image distribution of FIG. 16 is largely lowered as compared to that of FIG. 15. This shows that the image is extremely degraded if the light transmits through the optical system having an aberration.
It has been very difficult to realize an optical system having a high optical performance over an entire range of light when the light transmits through the optical system 101 twice. For example, Japanese Unexamined Patent Publication No. 5-332733 discloses a confocal detection method provided with a confocal optical system. However, since light from a measurement object transmits through the optical system twice, this method also has the aforementioned problem.
Further, in the case that the confocal optical system has a zooming function, an extremely highly precise optical design and a complicated and highly precise optical system are required in order to suppress the optical aberration to a low level over the entire zooming range.
Furthermore, in the confocal optical system 101 used in the conventional confocal detection method shown in FIG. 14, measurement light of various intensities such as specular reflection and diffuse reflection from the surface of the measurement object 100 are incident on the light receiving section 105. Accordingly, the variation range where the intensity of light incident on the light receiving section 105 becomes considerably wide, thereby sometimes exceeding the dynamic range of the light receiving section 105.
It is an object of the present invention to provide an apparatus and method for measuring a three-dimensional shape of an object which are free of the problems residing in the prior art.
According to an aspect of the present invention, a three-dimensional shape measuring apparatus comprises an illuminator for illuminating a measurement object by two luminous fluxes in different directions. The two luminous fluxes intersect each other at a specified position. The apparatus further comprises a light receiver including a plurality of photoelectric conversion elements for receiving light to generate an electric signal in accordance with an intensity of received light, and an objective optical system for transmitting light reflected from the measurement object to the light receiver.
According to another aspect of the present invention, a three-dimensional shape measuring apparatus comprises: a first measuring system; a second measuring system; a first controller for controlling the first measuring system to obtain first positional data about a measurement object; a measurement region calculator for calculating a measurement range based on obtained first positional data; and a second controller for controlling the second measuring system to obtain second positional data of the measurement object within a calculated measurement region.
According to still another aspect of the present invention, a method for measuring a three-dimensional shape of an object, comprises the steps of: obtaining first positional data of an object at a first measurement accuracy; determining a measurement region based on the obtained first positional data; obtaining second positional data of the object within the determined measurement region at a second measurement accuracy, the second measurement accuracy being higher than the first measurement accuracy; and calculating a three-dimensional shape based on the second positional data.
These and other object, features and advantages of the present invention will become apparent upon reading the following detailed description along with accompanying drawings.